The present invention relates to a method and system for detecting explosives and narcotics. More particularly, the invention relates to a nuclear quadrupole resonance (NQR) method and system for detecting target substances contained in explosives and narcotics.
There is a growing demand for more efficient and reliable systems for detecting and identifying substances such as explosives and narcotics. These systems are useful in a number of applications, including luggage or mail searches, land mine detection, and the control of illegal drug traffic, to name just a few.
It is recognized that NQR (nuclear quadrupole resonance) spectroscopy is a useful technique for detecting explosives and narcotics. While NQR is related to the better-known NMR (nuclear magnetic resonance) spectroscopic technique, NQR has a number of features that distinguish it from NMR.
NMR is based on the principle that: 1) a nucleus with non-zero total spin can be viewed as being characterized by an intrinsic net magnetic moment, which is polarized by a static external magnetic field B0; and 2) upon application of an oscillating RF magnetic field (at the correct resonant frequency) to the polarized nuclear magnetic moment, a resonance phenomenon occurs, resulting in the emission by the nucleus of a NMR response signal.
In NMR, the strength of the external static magnetic field B0 determines the amplitude and frequency of the NMR resonance signal. It is therefore advantageous to provide as strong an external magnetic field as possible. The strength of the B0-field in NMR systems is typically in the range of four or five orders of magnitude higher than the strength of the earth""s magnetic field. In order to induce nuclear magnetic resonance, a sample is typically placed inside a large magnet with a strong B0 field, to yield a net magnetization along the B0 field.
The typical range of NMR resonant frequencies is from about 10 MHz to about 500 MHz, depending on the B0 field strength and the specific nucleus. The NMR resonance frequencies shift slightly (on the order of several parts per million) for a given nucleus from one chemical bond to another. The external static B0-field must be quite homogeneous, in order to obtain a high-resolution spectrum that can provide useful information for studying molecular structure and other characteristics of interest. However, without knowing the compound within a sample, it is in general difficult to determine or identify a specific molecule from the NMR spectrum.
Nuclei that have a non-spherical charge distribution and a spin [xe2x89xa7] have, in addition to a magnetic moment, an electric quadrupole moment that is representative of the degree of deviation of the shape of a nucleus from a sphere. Nuclei having a non-zero electric quadrupole moment will be referred to in this application as xe2x80x9cquadrupolar nuclei.xe2x80x9d In NQR, the electric quadrupole moment of a quadrupolar nucleus interacts with the gradient of the local atomic electric field. This electrical interaction provides the mechanism for transitions between energy levels. However, these transitions, i.e. resonances, arc initiated and observed through the magnetic moment xcexc of the nucleus. Therefore, NQR measurements do not require the provision of a static external magnetic field.
In NQR, the electric field gradient (xe2x80x9cEFGxe2x80x9d) polarizes the electric quadrupole moments, and thus exerts torque on the quadrupolar nuclei. The nuclei precess (under a simplistic xe2x80x9cclassicalxe2x80x9d picture) about the EFG, as a result of the external torque and the angular momentum that the nuclei possess. The frequency of precession matches the energy differences between excited and non-excited nuclear states.
By analogy to NMR, the net magnetization of an object containing quadrupolar nuclei can be observed as an NQR response signal, by inducing excitations to higher energy nuclear quadrupole states through the application of an oscillating RF field. The oscillating RF field may be an RF pulse of finite duration, and has oscillation frequencies that match the frequencies of the allowed transitions between the different nuclear energy levels. As in NMR, NQR response signals are emitted as the nucleus returns to its original, lower energy state.
A number of quadrupolar nuclei are commonly present in many explosives and narcotics. These quadrupolar nuclei include, inter alia, nitrogen (14N, having spin I=1) and chlorine (35Cl and 37 Cl). In particular, nitrogen is present in most explosives, most explosives being nitrogen-containing crystalline solids. The NQR response signals for nitrogen nuclei range from several hundred kilohertz to a few megahertz. Typically, the NQR frequency range for most substances is less than about 6 MHz or so. In NQR, the strength of the EFG determines the NQR signal amplitude and frequency. In addition, the NQR signal amplitude also depends on the angle between the EFG and the RF field.
One advantage of NQR is high specificity: NQR response signals are very well-defined, i.e. are unique and specific to the particular type of molecule or compound being detected. A given quadrupolar nucleus sends out a distinctive NQR response signal, depending on the specific location of the nucleus within a molecular structure, as well on the specific molecule. Because of the wide variation in the local electric field gradient, the NQR frequencies differ considerably for a given nucleus situated in different locations within a given molecule, as well as in different molecules.
The NQR spectrum of a quadrupolar nucleus from a single type of molecule has multiple peaks, each peak being referred to as a spectral line. Generally, these spectral lines are at frequencies that are far away from each other, and also far away from the spectral lines of other types of molecules. Because of such a wide separation of the spectral lines, NQR provides a reliable method for identifying the specific quadrupolar nuclei, and for differentiating between chemical compounds that exhibit NQR responses. Because of their specificity, NQR spectral lines carry a distinctive signature for various types of narcotics, such as cocaine or heroine, and for various types of explosives containing nitrogen molecules.
Another advantage of NQR is that NQR measurements do not require an object to be placed in a strong, external magnetic field, as in NMR. In NMR, the sample is surrounded by static magnetic field coils and other gadgetries that produce the strong external B0 field. Therefore, the accessibility and configuration of NMR systems are limited. Also, most of the cost in NMR system is directed to the provision of strong, static magnetic fields. Therefore, NQR is more cost-effective, compared to NMR.
For these reasons, NQR provides an effective and practical technique for detecting explosives and narcotics. Despite these advantages, however, a major drawback of NQR measurements as performed in the prior art is that in typical situations, the NQR signal is inherently weak. It is necessary to tune the receiving RF coil to a narrow frequency range to minimize the noise. Even so, the NQR signal has to be collected repetitively and averaged over time, in order to achieve adequate signal-to-noise ratio for analysis and detection. If there is only one chemical to detect, the required time for such detection may not be too bad. However, in most cases, it is necessary to detect multiple chemical compounds. The required measurement time may become excessive, and thus the method may become impractical.
Accordingly, there is a need for an NQR detection system that has an improved efficiency, and thus allows for a reduced detection time. If many NQR spectral lines could be excited and detected at the same time, the total amount of measurement time could be greatly reduced. Further, by simultaneously collecting multiple NQR spectral lines of a chemical compound, the accuracy in identifying the chemical compound would be enhanced. It is therefore desirable to provide an NQR detection system that is capable of efficiently exciting and detecting multiple NQR spectral lines simultaneously.
The present invention provides a wideband NQR system that allows many spectral lines to be excited and detected at the same time. The wideband NQR system of the present invention includes a novel RF coil system having at least two RF coils that are shaped and configured to generate RF fields that are substantially de-coupled from each other. Each coil therefore can function completely independently, and the coil system can excite and detect multiple NQR spectral lines simultaneously.
A discovery of the present invention is that multiple RF coils can be used to perform simultaneous NQR excitation and detection over a given volume of an object that contains or conceals compounds that include one or more quadrupolar nuclei, if these RF coils are designed so as to generate RF magnetic fields that are substantially de-coupled from each other. For example, the RF fields may be substantially de-coupled by virtue of being mutually orthogonal. Because the RF fields generated by the RF coils are de-coupled, each RF coil can operate at different frequencies (or sets of frequencies), without any interference from each other. Each RF coil can be tuned to multiple resonant frequencies that match the characteristic transition frequencies of the NQR spectral lines of interest. Collectively, it is possible for the multiple RF coils to cover the entire frequency range for most, if not all, spectral lines of interest.
In one embodiment of the invention, a wideband NQR system for detecting NQR spectral lines includes a wideband transmitter for generating and transmitting pulsed RF signals. At least two RF coils are shaped and configured to generate, when driven by these RF signals, RF electromagnetic fields that are mutually orthogonal and substantially de-coupled, so that a plurality of nuclear quadrupole resonances are excited when the mutually orthogonal RF fields are applied to at least a portion of an object containing quadrupolar nuclei, at frequencies matching characteristic transition frequencies for the excited resonances. Preferably, the same de-coupled RF coils are used to detect the NQR response signals. The NQR system also includes a wideband receiver for receiving the detected NQR response signals.